Regulatory air pollution limits for diesel engines have caused heavy duty diesel truck manufacturers to adopt engine aftertreatment systems for treating diesel exhaust gases before release into the atmosphere. An aftertreatment system can include a plurality of catalytic units to reduce pollutants, including particulate soot (unburned hydrocarbons) and nitrogen oxide (NOx).
FIG. 1 shows a prior art aftertreatment exhaust system for a heavy duty diesel truck includes, among other components, a diesel particulate filter system 114 and a selective catalytic reduction system 116. The function of the diesel particulate filter system 114 is to reduce the particulates (soot), and the function of the selective catalytic reduction system 116 is to reduce nitrogen oxide.
The diesel particulate filter system 114 includes both a diesel particulate filter 108 and a diesel oxidation catalyst unit 106 ahead of the diesel particulate filter 108. The diesel particulate filter 108 traps particulates from the exhaust gas on a highly porous ceramic core, also referred to as a wall-flow filter. The filter 108 can undergo regeneration to convert the soot into carbon dioxide through chemical oxidation with an oxidant species. Heavy duty diesel truck manufacturers typically select nitrogen dioxide as opposed to oxygen to oxidize the soot, since oxidization with nitrogen dioxide generally proceeds at a lower temperature. However, while the exhaust gas generally includes a large amount of oxygen, the amount of nitrogen dioxide is relatively small. Accordingly, the diesel oxidation catalyst 106 can be used to convert nitrogen monoxide into nitrogen dioxide. In addition, the diesel oxidation catalyst unit 106 is used to remove residual hydrocarbons (HC) and convert carbon monoxide into carbon dioxide.
The exhaust gases, including the nitrogen dioxide from the diesel oxidation catalyst, pass into the diesel particulate filter 108, which traps soot. In addition to trapping soot, the diesel particulate filter 108 can include a catalyst to catalyze the oxidation of the soot with the nitrogen dioxide in a process of passive regeneration. Under some circumstances, the temperature of the exhaust gas alone may not be sufficient to initiate the oxidation reaction in passive regeneration. Accordingly, the temperature may be increased by using the hydrocarbon doser 104 to dose diesel fuel into the exhaust gases to raise the temperature and initiate oxidation, which is known as active regeneration.
The selective catalytic reduction system 112 includes a diesel exhaust fluid doser 110 and a selective catalytic reduction unit 112. The function of the selective catalytic reduction unit 112 is to convert NOx species into nitrogen (N2) and water through chemical reduction with a reductant species. The reductant species is usually ammonia. Ammonia is generated upon decomposition of urea, which is dosed as a solution via the diesel exhaust fluid doser 110. The selective catalytic reduction unit 112 can include a flow-through ceramic core loaded with a catalyst.
There have been attempts to reduce the components of the aftertreatment system by combining the functions of the diesel particulate filter 108 and the selective catalytic reduction unit 112 into a single unit, i.e., a diesel particulate filter with selective catalytic reduction capability. A diesel particulate filter that has capability for selective catalytic reduction of NOx is sometimes referred to as a selective catalytic reduction filter or SCRF. However, the conventional SCRFs have not been a viable alternative for heavy duty diesel applications, such as Class 8 trucks.